After drilling a hole through a subsurface formation and determining that the formation can yield an economically sufficient amount of oil or gas, a crew completes the well. During drilling, completion, and production maintenance, personnel routinely insert and/or extract devices such as tubing, tubes, pipes, rods, hollow cylinders, casing, conduit, collars, and duct into the well. For example, a service crew may use a workover or service rig to extract a string of tubing and sucker rods from a well that has been producing petroleum. The crew may inspect the extracted tubing and evaluate whether one or more sections of that tubing should be replaced due physical wear, thinning of the tubing wall, chemical attack, pitting, or another defect. The crew typically replaces sections that exhibit an unacceptable level of wear and notes other sections that are beginning to show wear and may need replacement at a subsequent service call.
As an alternative to manually inspecting tubing, the service crew may deploy an instrument to evaluate the tubing as the tubing is extracted from the well and/or inserted into the well. The instrument typically remains stationary at the wellhead, and the workover rig moves the tubing through the instrument's measurement zone.
The instrument typical measures pitting and wall thickness and can identify cracks in the tubing wall. Radiation, field strength (electrical, electromagnetic, or magnetic), sonic/ultrasonic pulses, and/or pressure differential may interrogate the tubing to evaluate these wear parameters. The instrument typically produces a raw analog signal and outputs a sampled or digital version of that analog signal.
In other words, the instrument, typically stimulates a section of the tubing using a field, radiation, or pressure and detects the tubing's interaction with or response to the stimulus. An element, such as a transducer, converts the response into an analog electrical signal. For example, the instrument may create a magnetic field into which the tubing is disposed, and the transducer may detect changes or perturbations in the field resulting from the presence of the tubing and any anomalies of that tubing.
The analog electrical signal output by the transducer can have an arbitrary or essentially unlimited number of states or measurement possibilities. That is, rather than having two discrete or binary levels, typical transducers produce signals that can assume any of numerous levels or values. As the tubing passes through the measurement field of the instrument, the analog transducer signal varies in response to variations and anomalies in the wall of the moving tubing.
The transducer and its associated electronics may have a dampened or lagging response that tends to reduce the responsiveness of the signal to tubing wall variations and/or noise. In other words, the instrument may acquire and process analog signals in a manner that steadies or stabilizes those analog signals. In typical conventional instruments, the analog processing remains fixed. That is, any damping or filtering of those signals is generally constant and inflexible.
The instrument also typically comprises a system, such as an analog-to-digital converter (“ADC”), that converts the analog transducer signal into one or more digital signals suited for reception and display by a computer. In conventional instruments, those digital signals typically provide a “snapshot” of the transducer signal. Thus, the ADC typically outputs a number, or set of a numbers, that represents or describes the analog transducer signal at a certain instant or moment in time. Since the analog transducer signal describes the section of tubing that is in the instrument's measurement zone, the digital signal is effectively a sample or a snapshot of a parameter-of-interest of that tubing section.
The analog-to-digital conversion typically occurs on a fixed-time basis, for example one, eight, or sixteen times per second. That is, conventional instruments usually acquire measurement samples at a predetermined rate or on a fixed time interval. Meanwhile, the speed of the tubing passing through the measurement zone often fluctuates or changes erratically. That is, the operator and rig may change the extraction speed in an unrepeatable fashion or in a manner that is not known in advance, a priori, or before the speed-change event.
Thus, the instrument may output a series of samples or digital snapshots with each sample separated by a tubing length that is not readily determined using conventional technology. The separation between samples might be a millimeter, a centimeter, or a meter of tubing length, for example. The distance between samples may vary, fluctuate, or change erratically as the operator changes the tubing speed. Moreover, the sample data may blur or become smeared when the tubing is moving rapidly. Consequently, fixing the time interval between each snapshot and allowing the tubing speed to vary between snapshots, as occurs in most conventional instruments, can produce data that is difficult to interpret or that fails to adequately characterize the tubing.
Another shortcoming of conventional instruments is that they generally provide an insufficient or limited level of processing of the digital samples. When the tubing is moving slowly through the instrument's measurement zone or is stationary, an operator may incorrectly interpret variation in the digital samples as a wall defect; however, the variation may actually result from signal noise. In other words, at slow tubing speeds, signal spikes due to noise or a random event can be mistaken for a defective tubing condition.
Meanwhile, when the tubing is moving quickly through the measurement zone, the tubing motion may blur or smooth signal spikes that are actually due to tubing defects, thereby hiding those defects from operator observation. That is, with conventional instruments, high-speed tubing motion may mask or obscure tubing wall defects. This phenomenon can be likened to the image blurring that can occur when a person takes a photograph of a fast moving car.
To address these representative deficiencies in the art, what is needed is an improved capability for evaluating tubing, for example in a petroleum application wherein the tubing is being placed into or drawn from an oil well. A further need exists for processing digital signals, samples, or snapshots of a physical parameter of the tubing. A further need exists for an instrument that can apply a flexible level of processing, filtering, or averaging to a signal from an instrument that is scanning or evaluating the tubing. Yet another need exists for processing instrumentation signals in a manner that smoothes noise while preserving signal structure indicative of valid tubing defects. Still another need exists for converting analog instrumentation or transducer signals into digital signals while accounting or compensating for changes in tubing speed. A capability addressing one or more of these needs would provide more accurate, precise, repeatable, efficient, or profitable tubing evaluations.